Mile-High Key Lime Pie

In bowl, sprinkle gelatin over 3 tablespoons juice; set aside. In pot, over medium high heat, bring milk and remaining juice to a boil. Remove from heat. In a medium bowl, whisk yolks together to blend. In thin, steady stream whisk 1/3 of milk mixture into yolks until combined. Over medium heat, cook mixture whisking, until thick enough to coat back of spoon, 5 minutes. Remove from heat; stir in zest, reserved gelatin mixture and if desired, add 3 to 5 drops green food coloring. Pour into bowl. Press plastic wrap against surface of custard; freeze or refrigerate until set, 2 to 4 hours. Beat cream with sugar until stiff. At medium speed beat custard until smooth; fold in 3 1/2 cups whipped cream. Set aside 2 cups filling; pour remaining into crust. Freeze pie and reserved filling 15 minutes. Top pie with reserved filling, mounding toward center. Spread or pipe remaining whipped cream over pie. Refrigerate until ready to serve. Makes 10 serving.